1. Field of the Invention
The present invention concerns: a method for adjustment of the position of a focus of an x-ray tube, the focus is unwontedly displaced by a magnetic interference field; and an x-ray tube for implementation of such a method, and a medical x-ray acquisition system with an x-ray tube operating according to such a method.
2. Description of the Prior Art
For some years, medical x-ray acquisition systems have been used in connection with magnetic navigation systems for improved control of catheters and guidance wires in blood vessels. Such an overall system composed of a medical x-ray acquisition system and magnetic navigation is known, for example, from the brochure “Axiom Artis dFC Magnetic Navigation” from Siemens Medical Solutions, 2004.
The magnetic field of the navigation system can lead to influences and disruptions of components of the medical x-ray acquisition system such as, for example, the x-ray tube. Typical x-ray tubes are known, for example, from A. Oppelt, “Imaging Systems for Medical Diagnostics”, Publicis Corporate Publishing, Erlangen, 2005, page 264 and the following, chapter 12.1, “The x-ray tube”. In particular an electron beam that is directed, accelerated onto a rotatable anode plate of the x-ray tube to generate x-ray radiation is very prone to external magnetic interference fields. The electron beam, whose impact point on the anode plate represents the focus point of the x-ray radiation, can be deflected from its intended path by such magnetic interference fields, such that it leads to an unwanted displacement of the position of the focus point and thus to an overall displacement of the generated x-ray beam.
The displacement of the focus position leads to a displacement of the region of the examination subject to be imaged by the x-ray image. Such a displacement is dependent on the distance of the examination subject from the focus, such that the displacement it is greater the closer that the examination subject is to the focus.
In order to reduce the effect of external magnetic interference fields, protective devices for x-ray tubes against are known in the form of ferromagnetic shieldings. Such devices have the disadvantage of exhibiting a high weight and magnetic field strengths in a magnetic field. Primarily in the case of an x-ray apparatus with a moving C-arm or a quickly-rotating gantry, the weight and the magnetic field strength can lead to mechanical deflections and out-of-balances.